there are two types of flowers, male and female flowers. If your flowers are on long slender stems, then they are male flowers. if they are on real short thicker stems, with a miniature pumpkin under it, then it's a female flower. Male flowers have the pollen. Female flowers don't. it's normal for the male flowers to only live one day. They only open in the morning, and close up in the afternoon. BTW you can eat pumpkin flowers.

if you do have female flowers you have to rely on bees to pollinate them, or do it yourself. See the thread on how to do that.